Researchers in the United States say animal tests on a vaccine against ricin have been successful.

Ricin is a natural toxin found in castor beans. It is cheap and relatively easy to produce. Just a small dose can kill. There is currently no approved vaccine.

It's cheap, simple and protects wonderfully without side effects

Dr Ellen Vitetta

Traces of ricin were found in caves in Afghanistan believed to have been used by al-Qaeda.

The toxin was also used to kill the Bulgarian dissident Georgi Markov in an infamous attack on London's Waterloo Bridge in 1978.

Cancer research

Dr Ellen Vitetta and colleagues at the University of Texas Southwestern Medical Center in Dallas developed the vaccine as a result of research into new anti-cancer drugs.

Many groups are carrying out studies to see if ricin can form the basis of anti-cancer treatment.

This is because the toxin can kill cells quite easily.

Ricin has two components - its 'B chain' binds to cells allowing the second component the so-called 'A chain' to enter the cell and kill it.

Dr Vitetta's team genetically engineered three versions of ricin's A chain. They found that two of these versions protected mice against the toxin.

The animals survived exposure to 10 times the dose of ricin that killed unvaccinated mice.

Dr Vitetta told New Scientist magazine that the vaccine also had no side-effects.

"It's cheap, simple and protects wonderfully without side effects because it's a totally inactive protein," she said.

According to the magazine, Dr Vitetta has now applied to the US National Institutes of Health for further funding to test the vaccine against aerosolised ricin with the hope of eventually testing the vaccine on people.

Her study was originally published in the journal Vaccine.

Professor Mike Lord, head of the toxin research group at the University of Warwick, is one of many scientists working in this area.

He told BBC News Online: "Because of the possible threat of ricin as a possible terrorist weapon a vaccine would be very useful.

"However, there is as yet no effective vaccine to protect against ricin."